Method of fabricating interconnect lines and plate electrodes of a storage capacitor in a semiconductor device

ABSTRACT

A method of fabricating a semiconductor device where the formation of a conductive layer typically over a storage capacitor on the device is used both as a plate electrode and also as an interconnect line. The method therefore combines the fabrication process steps of forming a plate electrode with the steps of forming a wiring layer. In a preferred embodiment, the storage capacitor is part of a cell array portion of a semiconductor memory device, whereas the interconnect line is in a peripheral portion of the memory device.

BACKGROUND OF THE INVENTION

The present invention is directed to a method of fabricatingsemiconductor devices. In particular, the invention is directed to amethod of fabricating interconnect lines together with plate electrodesof a storage capacitor in a semiconductor device such as a semiconductormemory.

The chosen method of fabricating interconnect lines or wiring layers ina semiconductor device plays an important role in the production ofsemiconductor devices such as semiconductor memories (e.g., dynamicrandom access memories (DRAM), static random access memories (SRAM),etc.). The manner in which such interconnect lines are fabricated in thedevice may affect the processing speed and reliability of the device, aswell as the manufacturing yield of the production.

With the constant pressure to achieve higher integration insemiconductor devices, especially in memory devices, the skilled artisanis required to forego the horizontal expansion of the integration acrossthe base of the device in favor of vertical development. This increasein vertical integration of a semiconductor device typically involvesapplying multiple layers of integration, thus, leading to an increase inthe aspect ratio (i.e., ratio of vertical height to the horizontallength) of the device. High aspect ratios lead to significant problemsin the fabrication process. Each layer added to the design of thesemiconductor device increases the complexity of and time delay inoverall production time.

FIGS. 1A-5B illustrate a conventional semiconductor device duringfabrication having a relatively high aspect ratio (approximately4.5-5.5). (For ease of illustration, among FIGS. 1A-5B, figures labeledwith like numbers represent the semiconductor device during the sameprocess step of fabrication. Figures having a suffix "A," however,represent a cross-sectional view of the cell array portion of aconventional semiconductor memory device viewed along line "A--A" ofFIG. 5A, whereas figures having a suffix "B," represent across-sectional view of the peripheral circuit portion of the samesemiconductor memory device viewed along line "B--B" of FIG. 5B.)

The conventional semiconductor memory device shown in Fig. 1A isfabricated with a source/drain region 2 and gate electrode 3 of atransistor formed on a semiconductor substrate 1. Over the electroderegion 3 an inter layer dielectric (ILD) is patterned having bit lines 4formed as shown in FIG. 1B. A storage electrode 5 (FIG. 1A) is patternedin and above the ILD. A dielectric film 6 is deposited over the storageelectrode 5, together with a plate electrode 7. The dielectric film 6and electrode 7 are patterned to complete a storage cell capacitor forthe memory cell in the cell array of the semiconductor memory device.

A planarization layer 8 (FIG. 2A) is used to planarize the step coverageproduced by the fabrication process. Material such as O₃ -TEOS (tetraethoxy silane) is typically used and deposited to a thickness of3000-7000 A (angstroms). An insulation layer 9, typically made ofPE-TEOS (plasma enhanced-tetra ethoxy silane), is deposited to athickness of 1000-3000 A. A layer of photoresist 10 is then formed andpatterned to allow for the formation of a contact hole in the peripheralportion of the semiconductor device, as shown in FIG. 2B. As shown inFIGS. 3A and 3B, the semiconductor device is then exposed to anisotropic-etching process step that etches insulation layer 9 andplanarization layer 8 to a depth of 1000-4000 A. The remainder of theplanarization layer 8 and ILD are then etched using ananisotropic-etching process to form contact hole 11 over source/drainregion 2 (FIG. 3B).

Tungsten is deposited in the contact hole 11 and on the top surface ofthe semiconductor device to a thickness of 2000-5000 A to form aconductive layer 12. This layer is patterned to form interconnect line12 in the peripheral portion of the semiconductor device, as shown inFIG. 4B. FIGS. 5A and 5B show mask works used during the fabricationprocess to form the various layers of the cell array portion and theperipheral portion, respectively, of the semiconductor memory device. Asshown in Fig. 5A, a gate electrode mask 3', a bit line mask 4', astorage electrode mask 5', and a plate electrode mask 7' arerespectively used to form the gate electrode 3, bit line 4, storageelectrode 5, and plate electrode 7 of the cell array portion of thesemiconductor device. Similarly, in FIG. 5B, gate electrode mask 3', bitline mask 4', contact hole mask 11', and interconnect line mask 12' arerespectively used to form the gate electrode 3, bit line 4, contact hole11, and interconnect line 12 of the peripheral portion of thesemiconductor device.

As can be seen from the above brief description of the fabricationprocess for a conventional semiconductor device, a semiconductor device,such as a memory device, having high integration requires a multitude oflayers. Any increase in integration, for example, adding interconnectlines or wiring layers, would increase the aspect ratio. As the aspectratio increases, the problems in fabrication and reliability of thedevice increase. For example, because the contact hole 11 (FIG. 3B) isso deep (i.e., 1.2 microns), the gas used to etch the contact hole inthe anisotropic-etching process step cannot react properly with theoxide layer at the bottom of the contact hole. As a result, the metalcontact necessary for the reliable operation of the semiconductor deviceis inferior. Furthermore, such an increase in the number of layerscauses significant delays in the overall production of the semiconductordevices as each new layer adds one or more steps to the overallfabrication process, thus, further complicating and delaying themanufacture of the devices.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method thatpermits the increase of integration of semiconductor devices without acorresponding increase in the aspect ratio of the devices.

It is an object of the invention to provide a method that allowsfabrication of highly integrated semiconductor devices without furthercomplicating or delaying the fabrication process.

The present invention accomplishes the above and other objects andadvantages by providing a method of fabricating a semiconductor devicewhere the formation of a conductive layer typically over a storagecapacitor on the device is used both as a plate electrode and also as aninterconnect line. The method therefore combines the fabrication processsteps of forming a plate electrode with the steps of forming a wiringlayer. In a preferred embodiment, the storage capacitor is part of acell array portion of a semiconductor memory device, whereas theinterconnect line is in a peripheral portion of the memory device.

Using the present invention, the high levels of integration desired insemiconductor devices can be achieved without increasing the aspectratio and without adding complex, time-consuming, and costly processsteps to the overall fabrication process. As a result, the contactinferiority and unreliability found in highly integrated semiconductordevices having high aspect ratios produced in accordance withconventional fabrication processes can be obviated. In addition, becausethe conductive layer of the semiconductor device fabricated inaccordance with the present invention functions as the plate electrodeof the storage capacitor in the cell array portion of the memory device,as well as functioning as the interconnect layer in the peripheralportion of the device, the added wiring layer does not require aseparate patterning process step. The increase in integration thereforedoes not require an accompanying increase in process steps andcorresponding process delays.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages of the present inventionwill become more apparent from the detailed description of the preferredembodiments of the present invention given below with reference to theaccompanying drawings in which:

FIGS. 1A and 1B respectively illustrate the cell array and peripheralcircuit portions of a conventional semiconductor memory device duringthe formation of the storage electrode in the cell array portion;

FIGS. 2A and 2B respectively illustrate the cell array and peripheralcircuit portions of a conventional semiconductor memory device duringthe deposition of a photoresist layer on the device;

FIGS. 3A and 3B respectively illustrate the cell array and peripheralcircuit portions of a conventional semiconductor memory device duringthe formation of a contact hole of the device;

FIGS. 4A and 4B respectively illustrate the cell array and peripheralcircuit portions of a conventional semiconductor memory device duringthe formation of an interconnect line of the device;

FIGS. 5A and 5B respectively illustrate mask works used to form the cellarray and peripheral circuit portions of a conventional semiconductormemory device;

FIGS. 6A and 6B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having a firstconductive layer formed in accordance with a first embodiment of thepresent invention;

FIGS. 7A and 7B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having a layer ofphotoresist deposited in accordance with a first embodiment of thepresent invention;

FIGS. 8A and 8B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having a secondconductive layer formed in accordance with a first embodiment of thepresent invention;

FIGS. 9A and 9B respectively illustrate mask works used to form the cellarray and peripheral circuit portions of a semiconductor memory devicein accordance with the process steps illustrated in FIGS. 6A-8B;

FIGS. 10A and 10B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having a firstconductive layer formed in accordance with a second embodiment of thepresent invention;

FIGS. 11A and 11B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having a layer ofphotoresist deposited in accordance with a second embodiment of thepresent invention;

FIGS. 12A and 12B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having a contact holeformed in accordance with a second embodiment of the present invention;

FIGS. 13A and 13B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having a secondconductive layer formed in accordance with a second embodiment of thepresent invention;

FIGS. 14A and 14B respectively illustrate mask works used to form thecell array and peripheral circuit portions of a semiconductor memorydevice in accordance with the process steps illustrated in FIGS.10A-13B;

FIGS. 15A and 15B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having a firstconductive layer formed in accordance with a third embodiment of thepresent invention;

FIGS. 16A and 16B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having a layer ofphotoresist deposited in accordance with a third embodiment of thepresent invention;

FIGS. 17A and 17B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having contact holesformed in accordance with a third embodiment of the present invention;

FIGS. 18A and 18B respectively illustrate the cell array and peripheralcircuit portions of a semiconductor memory device having a secondconductive layer formed in accordance with a third embodiment of thepresent invention; and

FIGS. 19A and 19B respectively illustrate mask works used to form thecell array and peripheral circuit portions of a semiconductor memorydevice in accordance with the process steps illustrated in FIGS.15A-18B.

DETAILED DESCRIPTION OF INVENTION

The present invention will be described in detail herein as embodied inthe preferred embodiments illustrated in FIGS. 6A-19B. Although theseembodiments depict the invention as applied to a semiconductor memorydevice, it should be readily apparent that the invention has equalapplication to any type of semiconductor device that encounters problemsrelated to an increase in vertical integration or can otherwise benefitfrom a reduction in the number of process steps required in fabrication.(For ease of illustration, among FIGS. 6A-19B, figures labeled with likenumbers represent a semiconductor device made in accordance with theinvention during the same process step of fabrication. Figures having asuffix "A," however, represent a cross-sectional view of the cell arrayportion of the semiconductor device as viewed along line "A--A" of oneof FIGS. 9A, 14A, or 19A, corresponding to the embodiment illustrated,whereas figures having a suffix "B," represent a cross-sectional view ofthe peripheral circuit portion of the same semiconductor memory deviceviewed along line "B--B" of one of FIGS. 9B, 14B, or 19B correspondingto the embodiment illustrated.)

FIGS. 6A and 6B illustrate a semiconductor device such as a DRAM devicehaving a cell array portion in which transistors are made ofsource/drain regions 2 within a base of silicon substrate 1, and gateelectrodes 3 deposited on substrate 1 above a channel region connectingadjacent or spaced apart source/drain regions 2. An inter layerdielectric (ILD) layer is formed over the transistors having bit lines 4deposited therein, as shown in FIG. 6B. The ILD may be formed, forexample, from annealing BPSG (boron phosphorous silicate glass) film inan atmosphere of nitrogen gas N₂) Any other known method of forming ILD,however, may suffice, including the use of HTO (high temperature oxide)film or USG (undoped silicate glass) film.

A storage electrode 5 is formed in the cell array portion of thesemiconductor memory device over the ILD. In the preferred embodiment,doped polysilicon having a thickness of 3000-7000 A (angstroms) is usedfor the storage electrode 5. (Any suitable dopant material, however,such as phosphorus, arsenic, boron or an equivalent may be used as adopant for the polysilicon used in the storage electrode.) The storageelectrode 5 is covered by a dielectric film 6 formed from Ta₂ O₅ with athickness of 50-150 A in the preferred embodiment. Other knowndielectric films deemed suitable, such as ONO (oxide nitride oxide), BZT(Ba_(x) Sr_(1-x) TiO₃), PZT (PbZrTiO₃), or other equivalent material maybe utilized.

A layer of polysilicon doped with ions of an element from Group V of theperiodic table (i.e., vanadium, niobium, tantalum, unnilpentium,nitrogen, phosphorus, arsenic, antimony, bismuth) is then formed asconductive layer 7A. (Although polysilicon is used to describe thisembodiment of the invention, it should be understood that any materialsuitable for use as a conductive layer, such as tungsten, can be used inpracticing the present invention.) The conductive layer 7A serves as theplate electrode of the storage capacitor in the cell array portion ofthe semiconductor memory device. Rather than terminating the conductivelayer 7A at the end of the cell array portion, the conductive layer 7Ais extended over the ILD in the peripheral circuit portion of thesemiconductor memory device.

As shown in FIGS. 7A and 7B, a layer of photoresist 10 is added andpatterned to begin formulation of contact hole 11. Using anisotropic-etching technique, an opening for the contact hole 11 isformed in conductive layer 7A. The ILD material is then anisotropicallyetched over the desired contact area, for example, on source/drainregion 2 of substrate 1, to complete the formation of contact hole 11.(In FIG. 7B, the contact hole 11 is illustrated as appearing over asource/drain region 2 in the peripheral portion of the semiconductordevice. It should be understood that the contact hole could appear inany area or portion of the semiconductor device without detracting fromthe present invention.)

To form an interconnect line for the semiconductor memory device, asuitable conductive material such as tungsten is deposited on theconductive layer 7A to form a second conductive layer 12, as shown inFIGS. 8A and 8B. In this preferred embodiment, the tungsten material isdeposited in contact hole 11 and on the surface of conductive layer 7Ato a thickness of 2000-8000 A. (Although tungsten is used in thispreferred embodiment as the additional conductive layer, it should beappreciated that any other suitable conductive material such as aluminummay be used in the practice of the invention.) Both conductive layer 7Aand second conductive layer 12 are exposed to an etching process stepthat concurrently patterns the two conductive layers to the desiredform, as shown in FIG. 8B.

FIGS. 9A and 9B show mask works used during the novel fabricationprocess to form the various layers of the cell array portion and theperipheral portion, respectively, of the semiconductor memory device. Asshown in FIG. 9A, a gate electrode mask 3', a bit line mask 4', astorage electrode mask 5', a plate electrode mask 7A', and aninterconnect line mask 12' are respectively used to form the gateelectrode 3, bit line 4, storage electrode 5, plate electrode 7A, andinterconnect line 12 of the cell array portion of the semiconductordevice. Similarly, in FIG. 9B, gate electrode mask 3', bit line mask 4',plate electrode mask 7A', contact hole mask 11', and interconnect linemask 12' are respectively used to form the gate electrode 3, bit line 4,plate electrode 7A, contact hole 11, and interconnect line 12 of theperipheral portion of the semiconductor device.

As can be seen from the description above, the semiconductor device,shown in FIGS. 6A-9B, fabricated in accordance with the presentinvention permits the addition of a wiring layer (i.e., interconnectline) in the peripheral portion of the semiconductor device withoutadding process steps to the overall fabrication process. Indeed, becausethe conductive layer 7A is used as both the plate electrode for thestorage electrode 5 in the cell array portion, and also as theinterconnect line in the peripheral portion of the semiconductor device,the separate photolithography steps of patterning a plate electrode andan interconnect line can be combined into one step. The increase inintegration of the device, therefore, does not necessitate the samecomplexity and delay inherent in the conventional fabrication processwhen integration is increased.

The invention also produces a semiconductor device having an aspectratio of 3 compared with the aspect ratios between 4.5 and 5.5 found indevices fabricated in accordance with conventional processes. The loweraspect ratio achieved through the present invention eliminates the needfor complex and costly fabrication technology without sacrificing higherintegration. Moreover, the semiconductor device resulting from theinventive fabrication process has a reduced contact hole depth (i.e.,less than 1 micron) due to the use of the conductive layer 7A in theperipheral portion as part of the interconnect line of the semiconductordevice. The smaller contact hole depth avoids the etching problems foundin conventional processes that produce devices having larger depths.

In another embodiment of the present invention, the process shown inFIGS. 6A-9B (described above) is modified in the following manner. Afterconductive layer 7A is deposited in the manner described in the previousembodiment, the conductive layer 7A is patterned as desired (FIG. 10Aand 10B). A planarization layer 8 is then deposited (FIGS. 11A and 11B)to a thickness of 3000-6000 A. (Although this embodiment employs O₃-TEOS as the planarization material, any other material suitable forplanarizing the surface of a substrate may be used in practicing theinvention. For example, the planarization layer 8 may be formed as aBPSG film subjected to an annealing process at temperatures of 800-900°C. in an atmosphere of N₂.) Insulation layer 9 is then deposited to athickness of 1000-3000 A using PE-TEOS (or other suitable insulationmaterial such as oxidation film). On the insulation layer 9 aphotoresist film 10 is deposited and patterned in order to form acontact hole in the peripheral portion of the semiconductor device.

In forming the contact hole 11 (FIG. 12B), the insulation layer 9 andplanarization layer 8 are isotropically etched to a depth of 1000-4000A. Anisotropically etching the remaining portion of planarization layer8 and the first conductive layer over the desired contact point, forexample, over source/drain region 2 of substrate 1, completes theformation of contact hole 11.

The contact hole 11 is filled with a conductive material such astungsten, aluminum, or other suitable material. This conductive materialis deposited in the hole 11 and on the surface of the device to athickness of 2000-8000 A to form a second conductive layer 12 (FIG.13B). This conductive layer 12 is then etched-back so that only thematerial within the contact hole 11 remains.

FIGS. 14A and 14B show mask works used during the novel fabricationprocess of this embodiment to form the various layers of the cell arrayportion and the peripheral portion, respectively, of the semiconductormemory device. As shown in FIG. 14A, a gate electrode mask 3', a bitline mask 4', a storage electrode mask 5', and plate electrode mask 7A'are respectively used to form the gate electrode 3, bit line 4, storageelectrode 5, and plate electrode 7A of the cell array portion of thesemiconductor device. Similarly, in FIG. 14B, gate electrode mask 3',bit line mask 4', plate electrode mask 7A', and contact hole mask 11',are respectively used to form the gate electrode 3, bit line 4, plateelectrode 7A, contact hole 11, of the peripheral portion of thesemiconductor device.

Using the process according to this embodiment, the interconnect line inthe peripheral portion of the semiconductor device has the secondconductive layer 12 in direct contact with source/drain region 2, and inside-wall contact with conductive layer 7A. Because the conductive layer7A is patterned for use as an interconnect line in the peripheralportion concurrently with the patterning of the layer 7A as a plateelectrode in the cell array portion, no additional metal patterningsteps are required to form the wiring layer of the semiconductor device.

In yet another embodiment of the invention, the same process stepsdiscussed above with respect to the other embodiments of the invention,namely the steps used to form source/drain regions 2 (FIGS. 15A and15B), gate electrodes 3, bit lines 4, the ILD, storage electrode 5,dielectric film 6, and conductive layer 7A, are duplicated in thisembodiment. The conductive layer 7A, however, is patterned so as topermit the formation of a wider metal contact area, as shown in FIG.15B. The conductive layer 7A continues to function both as the plateelectrode for storage electrode 5 in the cell array portion of thesemiconductor device, and also as the interconnect line for theperipheral portion of the device.

As shown in FIGS. 16A and 16B, a planarization layer 8, insulation layer9, and photoresist layer 10 are all deposited in the same manner as inthe previously described embodiments. The photoresist layer 10, however,is patterned in such a way as to facilitate the formation of twoadjacent contact holes. The two contact holes 11A and 11B (FIG. 17B) areformed through an etching process. First, an isotropic-etching processstep etches insulation layer 9 and planarizing layer 8 to a depth of1000-4000 A. Second, an anisotropic-etching process step etches theremaining portion of the planarization layer 8 and the ILD over thedesired contact point, for example, source/drain region 2 of the siliconsubstrate 1, to form contact hole 11A. For the formation of the secondcontact hole 11B, the anisotropic-etching process step forms an openingover a contact area on conductive layer 7A, as shown in FIG. 17B.

As in the previous embodiments, a conductive material such as tungsten,aluminum, or any other suitable material, is deposited in the contactholes 11A and 11B, and over the surface of the semiconductor device to athickness of 2000-8000 A to form second conductive layer 12. The secondconductive layer 12 is then etched back to leave a portion of layer 12covering contact holes 11A and 11B, as shown in FIGS. 18A and 18B.

FIGS. 19A and 19B show mask works used during the novel fabricationprocess to form the various layers of the cell array portion and theperipheral portion, respectively, of the semiconductor memory device. Asshown in FIG. 19A, a gate electrode mask 3', a bit line mask 4', astorage electrode mask 5', and plate electrode mask 7A' are respectivelyused to form the gate electrode 3, bit line 4, storage electrode 5, andplate electrode 7A of the cell array portion of the semiconductordevice. Similarly, in FIG. 19B, gate electrode mask 3', bit line mask4', plate electrode mask 7A', contact hole mask 11', and secondconductive layer mask 12' are respectively used to form the gateelectrode 3, bit line 4, plate electrode 7A, contact hole 11, and secondconductive layer 12 of the peripheral portion of the semiconductordevice.

Producing a semiconductor device using this embodiment of the inventionresults in a high integration device having a relatively low aspectratio. By patterning the conductive layer 7A as an interconnect line inthe peripheral portion of the semiconductor device, the metal contactcan be made wider than those produced using conventional processes. Thisadditional dimension reduces the aspect ratio, while permitting anincrease in the integration.

While the invention has been described in detail in connection with thepreferred embodiments known at the time, it should be readily understoodthat the invention is not limited to such disclosed embodiments. Rather,the invention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but are commensurate with the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method of forming a portion of a semiconductordevice, wherein the formed semiconductor device has at least one storagecell, the method comprising the steps of:forming a storage electrode inthe semiconductor device; forming a first conductive layer over thestorage electrode, wherein the storage electrode and the firstconductive layer comprise a storage cell capacitor for the semiconductordevice; forming a second conductive layer over the first conductivelayer; and concurrently patterning the first and second conductivelayers in the semiconductor device to form both a plate electrode overthe storage capacitor of the semiconductor device, and also aninterconnect line for the semiconductor memory device wherein the plateelectrode is disposed in a cell array portion of the semiconductormemory device and the interconnect line is disposed in a peripheralcircuit area in a peripheral portion of the semiconductor memory device.2. The method of claim 1 further comprising the steps of:preparing asilicon base to include source and drain regions spaced apart by achannel region in the silicon base; forming at least one gate electrodeoverlying the channel region; forming an inter layer dielectric (ILD)overlying the silicon base and at least one gate electrode; forming adielectric film overlying the storage electrode in the cell arrayportion of the semiconductor device, wherein the dielectric film isformed prior to said step of forming a first conductive layer, said stepof forming a first conductive layer comprising the substeps of forming afirst conductive layer over the dielectric film in the cell arrayportion of the semiconductor memory device and over the ILD in theperipheral circuit area of the semiconductor memory device, wherein thestorage electrode, ILD, dielectric film, and cell array portion of thefirst conductive layer complete a storage cell capacitor for thesemiconductor memory device; forming a patterned layer of photoresistover the first conductive layer in both the cell array and peripheralcircuit areas of the semiconductor memory device; and forming a contacthole in the peripheral portion of the semiconductor memory device,wherein said step of forming a second conductive layer comprises thesubstep of depositing a conductive material in the contact hole and onthe first conductive layer; and wherein said step of concurrentlypatterning the first and second conductive layers comprises the substepsof patterning the plate electrode over the storage capacitor in the cellarray portion of the semiconductor memory device, and also forming theinterconnect line area over the contact hole in the peripheral circuitarea of the semiconductor memory device.
 3. The method of claim 2,wherein said step of forming an inter layer dielectric (ILD) comprisesthe substep of annealing the semiconductor memory device using nitrogengas to form an ILD of boron phosphorous silicate glass (BPSG);andwherein the storage electrode is made of doped polysilicon, thedielectric film is made of Ta₂ O₅, the first conductive layer is made ofpolysilicon doped with ions from nitrogen, and the second conductivelayer is made of tungsten.
 4. The method of claim 2, wherein said stepof forming a contact hole comprises the substeps of isotropicallyetching the first conductive layer in the peripheral portion, andanisotropically etching the ILD over a contact point in the peripheralportion of the semiconductor memory device.